The stroke volume of the heart has been recognized as providing a useful signal to control the timing circuit of a demand-type cardiac pacer. In such a system, the pacer pulse generator will output stimulating pulses in accordance with the physiologic demand indicated by stroke volume changes in the patient's heart. In U.S. Pat. No. 4,686,987 to Salo, et al., entitled "Biomedical Method and Apparatus for Controlling the Administration of Therapy to a Patient in Response to Changes in Physiologic Demand", a biomedical apparatus capable of sensing changes in the heart's ventricular volume or stroke volume is disclosed. The apparatus changes the operating performance of the device as a function of stroke volume. The teachings of U.S. Pat. No. 4,686,987 are hereby incorporated by reference. Salo, et al. teaches that a relatively low frequency signal (under 5 KHz) is applied between spaced electrodes disposed in the heart. The beating action of the heart serves to modulate the signal due to changes in impedance being sensed between these or other electrodes within the heart. The modulated carrier signal is processed to remove R-Wave and other electrical artifacts and then demodulated to remove the carrier frequency component, leaving an envelope signal which is proportional to instantaneous ventricular volume. This envelope signal then contains stroke volume and ventricular volume information which can be used by the biomedical apparatus to vary its operating parameters. For example, a current proportional to changes in the stroke volume may be injected into the timing circuit of a demand-type cardiac pacer pulse generator whereby the interpulse interval of the pulse generator is varied as a function of stroke volume.
A copending application assigned to the assignee of this application having U.S. patent application Ser. No. 07/664,461, filed Mar. 1, 1991, and entitled "Variation In Cardiac Chamber Volume or Pressure as a Controlling Parameter", is also incorporated herein by reference. The aforereferenced application recognizes that the ventilatory signal also appears as a component of the impedance signal. Because the intrathoracic pressure is directly related to ventilation (i.e. pressure drops during inspiration and increases during expiration), the amplitude of the variation in intrathoracic pressure during a ventilatory cycle is directly related to the depth of ventilation (i.e. respiration). U.S. patent application Ser. No. 07/664,461 provides an impedance system for measurement of right ventricular (or atrial) volume or a pressure transducer for measurement of right ventricular (or atrial) pressure, a signal processing means to extract one of the volume or pressure parameters on a beat-by-beat basis to thereby yield a signal varying at the ventilatory rate and with a peak-to-peak amplitude proportional to ventilatory depth.
Referring again to the Salo, et al. patent, for example, a cardiac lead having two sensing electrodes and a stimulating electrode is used. Often, in the case of a cardiac pacer replacement, a bipolar lead having only two electrodes has previously been implanted in the heart. In such cases, since it is desirable to use the already implanted lead with a new pacemaker system in the case of, for example, replacing a worn-out pacemaker, the three electrode lead as used by Salo, et al. is often not available. In such cases, only three electrodes are typically available, namely, the pulse generator case or can, a lead ring on the endocardial lead and a tip electrode on the endocardial lead. Prior approaches to implementing an intracardiac impedance system with only three electrodes available have used at least one electrode as a simultaneous drive and sense electrode, since two drive and two sense points are required. Such approaches have several disadvantages.
One disadvantage of prior art techniques results from a high current density region being sensed at the "common" electrode (i.e., the electrode being used as both a drive and sense electrode) making it very sensitive to local effects such as, for example, mechanical motion. Another disadvantage of prior art systems results from the interface impedance at the common electrode which presents a large DC offset when sensed, yielding a lower modulation index relative to that experienced with tetrapolar impedance. Yet another drawback of prior art systems is that if the common electrode is on the pacemaker lead, either the ring or the tip, system performance will vary as a function of electrode material, effective surface area, geometry and various other electrode characteristics.
The method of the present invention uses tetrapolar impedance techniques and overcomes the above described disadvantages of prior art devices. Since the present invention effectively implements a tetrapolar impedance system that provides a stroke volume signal using any bipolar pacing lead, the quality of the sensed stroke volume signal equals that of a tetrapolar system using a pulse generator can and a tripolar pacing lead. In a further aspect, the signal sensed with the present invention contains a lower frequency component due to ventilation. This component may be extracted as it is related to tidal volume and may be used as another rate controlling parameter.
The present invention also affords an advantage even when used on a unipolar pacing lead. Although a tetrapolar method is not possible for intra-cardiac use in such a case, the dual indifferent method provided by the invention allows a tripolar technique. This has the advantages of reduced motion artifact at the pacer can, as well as a lower DC offset.